Shigeki Kawai,†,‡,* Baran Eren,†,§ Laurent Marot,† and Ernst Meyer†
ARTICLE
Graphene Synthesis via Thermal Polymerization of Aromatic Quinone Molecules †
Department of Physics, University of Basel, Klingbergstrasse 82, 4056 Basel, Switzerland, ‡PRESTO, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Japan, and §Material Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, United States
ABSTRACT Graphene was synthesized from pentacenequinone mol-
ecules on a Cu(111) surface using a three-step thermal treatment process: (1) self-assembly of a single layer molecular film at 190 °C, (2) formation of covalent bonding between adjacent molecules at intermediate temperatures, (3) thermal dehydrogenation and in-plane carbon diffusion at 600 °C. Transformation of the surface conformation was monitored with bimodal atomic force microscopy at the atomic scale and was corroborated with corelevel X-ray photoelectron spectroscopy. A strong CdO 3 3 3 HC hydrogen bonding involving the quinone moiety plays a key role in graphene growth, whereas conventional pentacene simply desorbs from the substrate during the same process. The most significant achievement of this proposed technique is obtaining graphene a couple of hundred degrees lower than standard techniques. Intrinsic defects due to carbon deficiency and the defects intentionally introduced by the microscope tip were also investigated with atomic-scale imaging. KEYWORDS: graphene . self-assembly . polymerization . bimodal AFM . XPS
E
ver since its first isolation achieved by mechanical exfoliation from a bulk graphite crystal,1 graphene has attracted tremendous scientific interest. Graphene not only possesses unusual electronic and mechanical properties such as high conductivity, quantum Hall effect and Berry's phase,2 massless Dirac fermions,3 and an elastic modulus of 1 TPa4 but also offers unique technological applications such as gas sensors,5 quantum computing elements, high-frequency transistors,6,7 and terahertz oscillators. Fabrication techniques are of central importance for harnessing properties of graphene in an optimum way. Considering growth on metals, two common ways can be described.810 The first way is via decomposition of hydrocarbons on metal surfaces with sufficient catalytic activity (i.e., platinum) and is classified as chemical vapor deposition (CVD). Among metal substrates, a polycrystalline Cu substrate (Cu foil) is the most attractive due to low-cost and large-scale production.11 Single-crystalline Cu substrates were also used in the literature for the CVD growth,12,13 where high-temperatures around 1000 °C KAWAI ET AL.
(very close to the melting point of Cu) were deemed necessary to form graphene in order to have a delicate balance between the sticking rate of hydrocarbon precursors and the dehydrogenation probability. The second pathway involves metal substrates with high carbon solubility at high temperatures (i.e., nickel), from which carbon precipitates to the surface during cool-down, leading to crystallization of the pure graphene phase. In most cases, temperatures above 700 °C are considered a prerequisite, and annealing at lower temperatures typically results in amorphous and/or poorly crystalline carbon films.14 Such high temperatures are undesirable for economical and environmental reasons; thus lowering the formation temperature is essential. Here, we present a simple yet elegant solution for growing graphene on Cu(111) via a thermal polymerization of a selfassembled film of 6,13-pentacenequinone (PQ, C22H12O2) molecules. Strong interaction of the carbonyl oxygen atom in a quinone moiety with the surface can prevent its desorption during annealing at 190 °C, enabling strong hydrogen bonding VOL. XXX
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* Address correspondence to [email protected]. Received for review February 21, 2014 and accepted May 29, 2014. Published online 10.1021/nn501047v C XXXX American Chemical Society
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ARTICLE Figure 1. (a) Chemical structure of the PQ molecule. (b) Three-dimensional topographic view of as-deposited PQ molecules on a Cu(111) surface forming needle-like multiple layers. Topographic images obtained after different annealing steps at (c) 190 °C, (d) 250 °C, (e) 340 °C, and (f) 600 °C. Measurement parameters: Afirst = 10 nm and Δffirst = 3.3 Hz in (b), Asecond = 1.0 nm and Δfsecond = 9.0 Hz in (c), Asecond = 1.0 nm and Δfsecond = 13.0 Hz in (d), Asecond = 1.0 nm and Δfsecond = 15.0 Hz in (e), and Afirst = 2.0 nm and Δffirst = 12.0 Hz in (f).
between oxygen and hydrogen of a neighboring molecule. Deoxygenation, dehydrogenation, and inplane carbon diffusion processes taking place during annealing at higher temperatures lead to covalent bonding between adjacent molecules. Consequently, the total molecular mass becomes large enough to inhibit desorption during annealing at elevated temperatures, eventually forming single-layer graphene graphene on Cu(111). Contrary to the standard CVD procedure, a lower formation temperature (600 °C) appeared to be sufficient. RESULTS AND DISCUSSION Transformation from a Self-Assembled Molecular Layer to Flat Films. Here, we show large-scale PQ structures on Cu(111) after each individual step. PQ molecules (Figure 1a) were in situ evaporated from a crucible of a Knudsen cell at 110 °C in ultrahigh vacuum (UHV) while the substrate was kept at room temperature (RT). UHV conditions were maintained during sample transfer and imaging. Figure 1b shows the topography of the as-deposited molecules obtained by noncontact atomic force microscopy (AFM) in the constant frequency mode at RT.15 In contrast to pentacene (C22H14) molecules, which assemble in a layer-by-layer bulk structure in a standing-packing manner,16 a needle-like KAWAI ET AL.
structure was formed on top of the first PQ molecular layer, in which PQ molecules lie flat on the Cu(111) surface. This observation is in good agreement with the previous macroscopic measurements performed by Salzmann et al.,17 and the mechanism to form the different assembly is related to the different electrostatic potential of PQ from pentacene due to the presence of a quinone moiety.18 PQ molecules in the bulk crystal configuration were desorbed from the substrate by annealing at 190 °C. Although this temperature is much higher than the sublimation temperature of PQ molecules in the crucible (∼110 °C), strong chemisorption on Cu(111) anchored PQ molecules in the first layer to the substrate, leaving a monolayer film behind, and thus the amount of deposited molecules in the initial step did not require high accuracy. The quality of the self-assembled molecular film also improved drastically after this thermal treatment, and the domain sizes became larger than 50 nm. Since the surface was no longer corrugated, small-amplitude operation with the second flexural mode of the cantilever was hereafter used to improve the spatial resolution in AFM topography images,19,20 except for Figure 1d. Figure 1c shows the monolayer film with a domain size of up to 60 nm 100 nm. The conformation of the self-assembly is in agreement with VOL. XXX
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ARTICLE Figure 2. (a) Topographic (upper panel) and high-resolution ΔfTR maps (lower panel) of the self-assembled PQ film, (b) partially polymerized film, and (c) graphene. Measurement parameters: Δfsecond = 22.5 Hz, Asecond = 1.0 nm, and ATR = 30 pm in (a), Δfsecond = 37.0 Hz, Asecond = 1.0 nm, and ATR = 30 pm in (b), Δfsecond = 240 Hz Asecond = 500 pm, and ATR = 30 pm in (c).
the previous scanning tunneling microscopy (STM) study,21 as the carbonyl oxygen atom in the quinone moiety and a hydrogen atom of the adjacent molecule interacted in terms of strong CdO 3 3 3 HC bonding on the surface.18 Disordered zones started to appear at domain boundaries of the self-assembled film after annealing at 250 °C (Figure 1d), suggesting that the first chemical reaction of molecules on Cu(111) took place. An activation energy is surmounted around this temperature, leading to a catalytic reaction between the carbonyl oxygen atom and a hydrogen atom from the adjacent molecule as a result of their affinity to each other, which was also shown to prevail in the conformation prior to this step. Combining with residual hydrogen in the UHV chamber, water molecules were likely to be formed and then released to the vacuum during this step. The binding energy of PQ molecules in the ordered zones to the Cu(111) substrate was high enough for them to remain attached to the surface at 250 °C. This is not a necessity for the disordered zones, since PQ molecules cannot be desorbed anymore even at higher temperatures once polymerization starts. At 340 °C, the self-assembled structure had completely vanished, and a single disordered hydrocarbon polymer monolayer was obtained (Figure 1e). The temperature of the final annealing step was 600 °C, at which only flat islands separated by grooves were observed (Figure 1f). Furthermore, some of the islands indicated by an arrow are about 200 pm higher than KAWAI ET AL.
the rest, associated with the second layer. It is noteworthy to mention here that dehydrogenation presumably had already started at temperatures lower than 600 °C;22 however no significant effect had been observed since the carbon mobility was limited at these temperatures. Atomic-Scale Measurements. Transition of the structure at atomic scale was studied by high-resolution imaging. In order to achieve high-resolution images of the films, we used the recently developed bimodal AFM technique,23 in which two resonance modes of the cantilever were simultaneously employed.24,25 In this technique, the frequency shift of the vertical oscillation was used to control the tipsample Z distance (topographic image), and that of the torsional oscillation ΔfTR ensured high-resolution imaging.26,27 Figure 2a shows the topographic image of the initial self-assembled PQ film on the Cu(111) surface. The inset shows a plausible conformation, where the molecules self-assemble in such a way as to form the CdO 3 3 3 H hydrogen bonds with the adjacent molecules. The conformation of the PQ assembly is very similar to that previously reported on MoS2 by Strohmaier et al.21 In addition, we observed the superstructure, in which characteristic bright zones run in every three molecules. Since this feature was observed with a different measurement set point, it cannot be related to an imaging artifact. Presumably, the strong chemisorption of the molecules to the Cu substrate and its resultant commensurability induce this superstructure. VOL. XXX
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The lower panel shows the corresponding ΔfTR map. Since the direction of the tip apex movement by the torsional vibration of the cantilever is along the X direction (inset), the ΔfTR map is related to the lateral force gradient, which is the second derivative of the potential map along the X direction. The site-independent tipsample interaction causes no lateral force gradient, so that the detected ΔfTR is only related to the site-dependent interaction. On an atomically flat surface, the short-range tipsample interaction is responsible for the contrast and atoms are usually observed as more positive ΔfTR sites. However, on molecular films, middle-range tipmolecular interactions can also contribute to ΔfTR. This imaging mechanism makes the interpretation of the contrast more complicated compared to a conventional topography obtained by the vertical tipsample interaction. Furthermore, two oxygen atoms in the PQ molecule bind to the Cu substrate, resulting in a V-shaped bending configuration of the molecule such that both longitudinal ends reside facing up. These complexities induced the peculiar contrast shown in the lower panel of Figure 2a; nonetheless, clear periodic patterns with submolecular resolution are clear evidence of the presence of a highly ordered molecular film. Annealing at 250 °C induced partial polymerization of PQ molecules and introduced a flake-like structure in the topography and some new patterns with the periodicity of the lattice parameter of graphene in the ΔfTR map (Figure 2b), but without any long-range order (domain size less than a few nanometers). The remaining line structures are most probably related to PQ molecules, which were not yet covalently bonded to each other. After annealing at 600 °C, the topographic image in the island becomes very flat (
ARTICLE
Graphene Synthesis via Thermal Polymerization of Aromatic Quinone Molecules †
Department of Physics, University of Basel, Klingbergstrasse 82, 4056 Basel, Switzerland, ‡PRESTO, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Japan, and §Material Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, United States
ABSTRACT Graphene was synthesized from pentacenequinone mol-
ecules on a Cu(111) surface using a three-step thermal treatment process: (1) self-assembly of a single layer molecular film at 190 °C, (2) formation of covalent bonding between adjacent molecules at intermediate temperatures, (3) thermal dehydrogenation and in-plane carbon diffusion at 600 °C. Transformation of the surface conformation was monitored with bimodal atomic force microscopy at the atomic scale and was corroborated with corelevel X-ray photoelectron spectroscopy. A strong CdO 3 3 3 HC hydrogen bonding involving the quinone moiety plays a key role in graphene growth, whereas conventional pentacene simply desorbs from the substrate during the same process. The most significant achievement of this proposed technique is obtaining graphene a couple of hundred degrees lower than standard techniques. Intrinsic defects due to carbon deficiency and the defects intentionally introduced by the microscope tip were also investigated with atomic-scale imaging. KEYWORDS: graphene . self-assembly . polymerization . bimodal AFM . XPS
E
ver since its first isolation achieved by mechanical exfoliation from a bulk graphite crystal,1 graphene has attracted tremendous scientific interest. Graphene not only possesses unusual electronic and mechanical properties such as high conductivity, quantum Hall effect and Berry's phase,2 massless Dirac fermions,3 and an elastic modulus of 1 TPa4 but also offers unique technological applications such as gas sensors,5 quantum computing elements, high-frequency transistors,6,7 and terahertz oscillators. Fabrication techniques are of central importance for harnessing properties of graphene in an optimum way. Considering growth on metals, two common ways can be described.810 The first way is via decomposition of hydrocarbons on metal surfaces with sufficient catalytic activity (i.e., platinum) and is classified as chemical vapor deposition (CVD). Among metal substrates, a polycrystalline Cu substrate (Cu foil) is the most attractive due to low-cost and large-scale production.11 Single-crystalline Cu substrates were also used in the literature for the CVD growth,12,13 where high-temperatures around 1000 °C KAWAI ET AL.
(very close to the melting point of Cu) were deemed necessary to form graphene in order to have a delicate balance between the sticking rate of hydrocarbon precursors and the dehydrogenation probability. The second pathway involves metal substrates with high carbon solubility at high temperatures (i.e., nickel), from which carbon precipitates to the surface during cool-down, leading to crystallization of the pure graphene phase. In most cases, temperatures above 700 °C are considered a prerequisite, and annealing at lower temperatures typically results in amorphous and/or poorly crystalline carbon films.14 Such high temperatures are undesirable for economical and environmental reasons; thus lowering the formation temperature is essential. Here, we present a simple yet elegant solution for growing graphene on Cu(111) via a thermal polymerization of a selfassembled film of 6,13-pentacenequinone (PQ, C22H12O2) molecules. Strong interaction of the carbonyl oxygen atom in a quinone moiety with the surface can prevent its desorption during annealing at 190 °C, enabling strong hydrogen bonding VOL. XXX
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NO. XX
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* Address correspondence to [email protected]. Received for review February 21, 2014 and accepted May 29, 2014. Published online 10.1021/nn501047v C XXXX American Chemical Society
000–000
’
XXXX
A www.acsnano.org
ARTICLE Figure 1. (a) Chemical structure of the PQ molecule. (b) Three-dimensional topographic view of as-deposited PQ molecules on a Cu(111) surface forming needle-like multiple layers. Topographic images obtained after different annealing steps at (c) 190 °C, (d) 250 °C, (e) 340 °C, and (f) 600 °C. Measurement parameters: Afirst = 10 nm and Δffirst = 3.3 Hz in (b), Asecond = 1.0 nm and Δfsecond = 9.0 Hz in (c), Asecond = 1.0 nm and Δfsecond = 13.0 Hz in (d), Asecond = 1.0 nm and Δfsecond = 15.0 Hz in (e), and Afirst = 2.0 nm and Δffirst = 12.0 Hz in (f).
between oxygen and hydrogen of a neighboring molecule. Deoxygenation, dehydrogenation, and inplane carbon diffusion processes taking place during annealing at higher temperatures lead to covalent bonding between adjacent molecules. Consequently, the total molecular mass becomes large enough to inhibit desorption during annealing at elevated temperatures, eventually forming single-layer graphene graphene on Cu(111). Contrary to the standard CVD procedure, a lower formation temperature (600 °C) appeared to be sufficient. RESULTS AND DISCUSSION Transformation from a Self-Assembled Molecular Layer to Flat Films. Here, we show large-scale PQ structures on Cu(111) after each individual step. PQ molecules (Figure 1a) were in situ evaporated from a crucible of a Knudsen cell at 110 °C in ultrahigh vacuum (UHV) while the substrate was kept at room temperature (RT). UHV conditions were maintained during sample transfer and imaging. Figure 1b shows the topography of the as-deposited molecules obtained by noncontact atomic force microscopy (AFM) in the constant frequency mode at RT.15 In contrast to pentacene (C22H14) molecules, which assemble in a layer-by-layer bulk structure in a standing-packing manner,16 a needle-like KAWAI ET AL.
structure was formed on top of the first PQ molecular layer, in which PQ molecules lie flat on the Cu(111) surface. This observation is in good agreement with the previous macroscopic measurements performed by Salzmann et al.,17 and the mechanism to form the different assembly is related to the different electrostatic potential of PQ from pentacene due to the presence of a quinone moiety.18 PQ molecules in the bulk crystal configuration were desorbed from the substrate by annealing at 190 °C. Although this temperature is much higher than the sublimation temperature of PQ molecules in the crucible (∼110 °C), strong chemisorption on Cu(111) anchored PQ molecules in the first layer to the substrate, leaving a monolayer film behind, and thus the amount of deposited molecules in the initial step did not require high accuracy. The quality of the self-assembled molecular film also improved drastically after this thermal treatment, and the domain sizes became larger than 50 nm. Since the surface was no longer corrugated, small-amplitude operation with the second flexural mode of the cantilever was hereafter used to improve the spatial resolution in AFM topography images,19,20 except for Figure 1d. Figure 1c shows the monolayer film with a domain size of up to 60 nm 100 nm. The conformation of the self-assembly is in agreement with VOL. XXX
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ARTICLE Figure 2. (a) Topographic (upper panel) and high-resolution ΔfTR maps (lower panel) of the self-assembled PQ film, (b) partially polymerized film, and (c) graphene. Measurement parameters: Δfsecond = 22.5 Hz, Asecond = 1.0 nm, and ATR = 30 pm in (a), Δfsecond = 37.0 Hz, Asecond = 1.0 nm, and ATR = 30 pm in (b), Δfsecond = 240 Hz Asecond = 500 pm, and ATR = 30 pm in (c).
the previous scanning tunneling microscopy (STM) study,21 as the carbonyl oxygen atom in the quinone moiety and a hydrogen atom of the adjacent molecule interacted in terms of strong CdO 3 3 3 HC bonding on the surface.18 Disordered zones started to appear at domain boundaries of the self-assembled film after annealing at 250 °C (Figure 1d), suggesting that the first chemical reaction of molecules on Cu(111) took place. An activation energy is surmounted around this temperature, leading to a catalytic reaction between the carbonyl oxygen atom and a hydrogen atom from the adjacent molecule as a result of their affinity to each other, which was also shown to prevail in the conformation prior to this step. Combining with residual hydrogen in the UHV chamber, water molecules were likely to be formed and then released to the vacuum during this step. The binding energy of PQ molecules in the ordered zones to the Cu(111) substrate was high enough for them to remain attached to the surface at 250 °C. This is not a necessity for the disordered zones, since PQ molecules cannot be desorbed anymore even at higher temperatures once polymerization starts. At 340 °C, the self-assembled structure had completely vanished, and a single disordered hydrocarbon polymer monolayer was obtained (Figure 1e). The temperature of the final annealing step was 600 °C, at which only flat islands separated by grooves were observed (Figure 1f). Furthermore, some of the islands indicated by an arrow are about 200 pm higher than KAWAI ET AL.
the rest, associated with the second layer. It is noteworthy to mention here that dehydrogenation presumably had already started at temperatures lower than 600 °C;22 however no significant effect had been observed since the carbon mobility was limited at these temperatures. Atomic-Scale Measurements. Transition of the structure at atomic scale was studied by high-resolution imaging. In order to achieve high-resolution images of the films, we used the recently developed bimodal AFM technique,23 in which two resonance modes of the cantilever were simultaneously employed.24,25 In this technique, the frequency shift of the vertical oscillation was used to control the tipsample Z distance (topographic image), and that of the torsional oscillation ΔfTR ensured high-resolution imaging.26,27 Figure 2a shows the topographic image of the initial self-assembled PQ film on the Cu(111) surface. The inset shows a plausible conformation, where the molecules self-assemble in such a way as to form the CdO 3 3 3 H hydrogen bonds with the adjacent molecules. The conformation of the PQ assembly is very similar to that previously reported on MoS2 by Strohmaier et al.21 In addition, we observed the superstructure, in which characteristic bright zones run in every three molecules. Since this feature was observed with a different measurement set point, it cannot be related to an imaging artifact. Presumably, the strong chemisorption of the molecules to the Cu substrate and its resultant commensurability induce this superstructure. VOL. XXX
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ARTICLE
The lower panel shows the corresponding ΔfTR map. Since the direction of the tip apex movement by the torsional vibration of the cantilever is along the X direction (inset), the ΔfTR map is related to the lateral force gradient, which is the second derivative of the potential map along the X direction. The site-independent tipsample interaction causes no lateral force gradient, so that the detected ΔfTR is only related to the site-dependent interaction. On an atomically flat surface, the short-range tipsample interaction is responsible for the contrast and atoms are usually observed as more positive ΔfTR sites. However, on molecular films, middle-range tipmolecular interactions can also contribute to ΔfTR. This imaging mechanism makes the interpretation of the contrast more complicated compared to a conventional topography obtained by the vertical tipsample interaction. Furthermore, two oxygen atoms in the PQ molecule bind to the Cu substrate, resulting in a V-shaped bending configuration of the molecule such that both longitudinal ends reside facing up. These complexities induced the peculiar contrast shown in the lower panel of Figure 2a; nonetheless, clear periodic patterns with submolecular resolution are clear evidence of the presence of a highly ordered molecular film. Annealing at 250 °C induced partial polymerization of PQ molecules and introduced a flake-like structure in the topography and some new patterns with the periodicity of the lattice parameter of graphene in the ΔfTR map (Figure 2b), but without any long-range order (domain size less than a few nanometers). The remaining line structures are most probably related to PQ molecules, which were not yet covalently bonded to each other. After annealing at 600 °C, the topographic image in the island becomes very flat (
